1. Field of the Invention
The invention relates to a combined pressure atomizing nozzle operated with liquid fuel and intended for gas-turbine burners according to the preamble of claim 1.
2. Discussion of Background
Low-pollution combustion of liquid fuels, such as, for example, extra-light fuel oil, requires the complete vaporization of the fuel droplets and the premixing of the fuel vapor with the combustion air before the flame front is reached. Even small zones of higher fuel concentration lead to elevated temperatures in the reaction zone and thus to intensified formation of thermal nitrogen oxides. A disadvantage of lean premixed flames is that the flame temperatures lie very close to the extinction limit. In order to realize a burner operation which continues to be stable at low load and thus lower flame temperature, specific enrichment of the flame-stabilization zones is necessary. There is therefore the problem of covering a wide operating range of the gas turbine with a burner and an atomizing nozzle.
That depth of penetration of the fuel spray into the combustion air which is required for a good distribution of the fuel in the combustion air is influenced in particular by the ratio of the impulse flows of combustion air and fuel. This ratio changes with the operating conditions, i.e. as a result of changes in the fuel mass flow, in the fuel pressure, and in the temperature and the pressure of the burner air. The vaporization time of the fuel depends essentially on the atomizing quality, the relative velocity between fuel and air, and the ambient boundary conditions such as temperature and pressure. Whereas the latter are predetermined for the different load states by the gas-turbine process, the atomizing quality and the relative velocity are mainly determined by the atomizing nozzle. In conventional gas-turbine burners, spill-controlled swirl atomizers or two-stage swirl atomizers are used in order to compensate for the changing ambient conditions. However, since no specific change in the spraying direction is possible in swirl nozzles, these known atomizing nozzles merely permit a rough adaptation of the atomizing quality and the fuel impulse to the respective load conditions.
An improvement can be achieved with the high-pressure atomizing nozzle disclosed in EP-A2-0 711 953, the discharge orifices of which are oriented to the zones of high air velocity and in which the angle of the fuel spray to the axis of the burner is at least as large as the cone half angle of the burner. As the name reveals, high pressure is required to operate this pressure atomizing nozzle, which is suitable in particular for a double-cone burner disclosed by EP-B1-0321 809. To this end, the liquid fuel must be fed at a pressure of more than 100 bar, which, however, requires a considerable design input for the fuel system. In addition, a change in the spraying direction or in the jet profile is not possible either.
German Patent 862 599 discloses a combined two-stage or multi-stage swirl atomizer, which, however, has an impulse behavior which is unsuitable for gas-turbine burners. Although very fine atomization is achieved with the resulting swirl spray, the fuel impulse is too low to achieve an adequate distribution of the fuel droplets in the combustion air and thus good premixing.
Accordingly, one object of the invention, in attempting to avoid these disadvantages, is to provide a novel combined pressure atomizing nozzle for gas-turbine burners, with which an improved adaptation of the atomizing quality of liquids to the respective load conditions, i.e. good premixing over the entire load range, can be realized.
According to the invention, this is achieved in that, in a device according to the preamble of claim 1, the second feed passage has at least two discharge orifices to the outer space. As a result, the combined pressure atomizing nozzle is designed as a multi-hole diaphragm nozzle having a simple, central nozzle, which, in addition to the fine atomization of the liquid fuel, also ensures a high burner impulse. In this way, both quick vaporization of the liquid fuel and good premixing of the fuel spray with the combustion air can be achieved, for which reason the pressure atomizing nozzle according to the invention is also suitable in particular for gas-turbine burners. In addition, a pressure atomizing nozzle of relatively simple construction and having a small space requirement is provided, the two-stage arrangement of which is realized only by the additional inclusion of the discharge orifices of the second feed passage.
Compared with the spraying of the liquid fuel via an annular duct (German Patent 862 599), which spraying is disclosed by the prior art, a plurality of separate fuel sprays having a relatively narrow spray cone are produced by the discharge orifices according to the invention. However, these fuel sprays have a markedly higher impulse than an annular fuel spray and also have a higher velocity of the liquid fuel relative to the combustion air. Improved premixing can therefore be achieved with this solution. When the fuel distribution between the diaphragm nozzle and the central nozzle corresponds to the actual operating situation, both specific intermixing between the liquid fuel and the combustion air and an adaptation of the fuel impulse to the requisite depth of penetration of the liquid fuel into the combustion air are made possible with the pressure atomizing nozzle. Such a mass flow of liquid which corresponds to the requisite fuel quantity of the gas turbine at part load is fed to the pressure atomizing nozzle via the central nozzle, i.e. through the first feed passage.
It is especially advantageous if the discharge orifices of the second feed passage are uniformly distributed over the periphery of the nozzle body. This arrangement ensures a uniform fuel concentration in the reaction zone and therefore prevents the intensified formation of nitrogen oxides.
The first feed passage is formed in the interior of a first tube, and the second feed passage is formed in the interior of a second tube. Both tubes are arranged concentrically to one another and are closed off from the outer space downstream by a cover. The cover and the first tube are designed in one piece. As a result, the pressure atomizing nozzle can be assembled in a relatively simple manner by the second tube being pushed onto the first tube up to its stop at the cover. The second tube and the cover are then firmly connected to one another, for example by welding.
A turbulence chamber is advantageously formed directly upstream of the discharge orifices of the second feed passage. The atomization of the liquid fuel can be improved by the additional inclusion of the turbulence chamber in the multi-hole diaphragm nozzle. The turbulence chamber is separated from the second feed passage by a partition. At least two turbulence-generating orifices are arranged in the partition eccentrically with respect to the second feed passage. With such asymmetrical directing of the liquid fuel into the turbulence chamber, higher turbulence can be achieved and, as a result, the atomization of the liquid fuel can be further improved.
In an especially advantageous manner, the turbulence-generating orifices are arranged offset from the discharge orifices of the second feed passage. In this case, the offset, in the case of four turbulence-generating orifices or discharge orifices respectively, is preferably about 45xc2x0, so that the turbulence-generating orifices are arranged exactly centrally between the discharge orifices. This leads to a more intensive, small-scale and turbulent structure, i.e. to a very fine fuel spray.
The pressure atomizing nozzle constructed with the additional turbulence chamber can likewise be assembled in a relatively simple manner. To this end, the cover, the first tube and the partition are designed in one piece, so that these components can be inserted together, as an insert so to speak, into the second tube. Finally, the first tube and the cover are firmly connected to one another, for example by welding.
As an alternative to a simple, central nozzle, either a swirl chamber or a turbulence chamber is formed between the first feed passage and the discharge orifice. In the first case, i.e. when using a swirl nozzle, a swirl spray having a relatively wide spray cone is produced, so that a high fuel concentration in the center of the burner as well as sufficient vaporization of the fuel can also be achieved at part load. This also permits a stable burner operation in the part-load range of the gas turbine. If, on the other hand, a turbulence nozzle is used as central nozzle, a narrower spray angle can be realized while the atomization of the liquid fuel remains equally effective. In this way, the fuel concentration in the center of the burner can be further increased and the burner operation can thereby be additionally stabilized at part load.